On the Improvement of the Fatigue Behaviour of Austenitic Stainless Steels Due to Surface Residual Stresses Produced by Low Temperature Carburizing

Low temperature carburizing (LTC) treatment improves surface hardness and wear resistance of the austenitic stainless steels without reducing their corrosion resistance. Surface hardness over 1000 Vickers and compressive residual stresses whose modulus exceeds 1500 MPa are usually achieved in the carburized layer, thanks to the formation of the so-called “S-phase”, a carbonsupersaturated austenite phase. A significant enhancement of the fatigue resistance due to different versions of this process is reported in literature. The achievements obtained in recent years on the subject are summarized in this paper and the results of new rotating bending fatigue tests are illustrated. These tests showed that the low temperature carburizing treatment enhances the fatigue strength of the solution annealed 316 steel by 26.4% with respect to the non-treated material due to the high residual stresses present in the treated layer. A much higher data spread than the one of the non-treated material was found and its causes have to be further investigated. Fatigue cracks in the surface-treated specimens always nucleated under the surface, near the boundary between the carburized case and the core. A critical analysis of the recent literature on the modelling of fatigue behaviour of metals in the presence of surface compressive residual stresses was performed in order to verify the applicability of the models' hypotheses to the LTC case.